Why is Luigi terrifying and out for blood?

Since the debut of Mario Kart 8 on May 30th, many video game fans noticed that the awkward little brother of Nintendo's famous mascot, Mario, seemed to have a serious axe to grind with the rest of the video game's cast. In the game, whenever passing or striking another player's car, Luigi slowly turns his head to reveal a death glare so cold-blooded, it changes our entire perspective of his character. So why has the once adorable little plumber's assistant gone full-blown psychopath? Let's look at the evidence at hand.